Figure 16.2 Global distribution of water stress index, where dark red indicates a region of severe water stress (Oki and Kanae 2006).

Figure 16.2 Global distribution of water stress index, where dark red indicates a region of severe water stress (Oki and Kanae 2006).

The geographical distribution of human demand for water does not necessarily coincide with that of water availability. A commonly displayed global water resources assessment plot (Figure 16.2) shows clearly the difference between these two distributions. The map illustrates the distribution of a water scarcity index R = W/Q, where W is the water withdrawal and Q is the renewable freshwater. When R is higher than 0.4, the region is considered to be an area of high water stress (displayed in dark red in the figure). Northern China, Central Asia, northern India and Pakistan, parts of the Middle East, and the middle to western U.S. are among these regions. More than two billion people are estimated to live in high water stress regions, where lower sustainability may be anticipated.

Starting from the geographical imbalance between water availability and water withdrawal/use/demand, I will address the potential for quantitatively measuring and monitoring world water sustainability. Critical issues are discussed; however, the quality of water will not be addressed. For a discussion of water quality, see Knepper and Ternes (this volume).

Water Stress and Its Relation to (Un-)Sustainability

High water stress would not appear if the geographical distribution of water withdrawal/demand coincided with that of water availability. Why, then, is there such a high human demand for water in regions with relatively low water availability?

First, agriculture (specifically for irrigation purposes) is responsible for approximately 70% of the water withdrawal. The majority of water withdrawn for agricultural purposes evaporates into the atmosphere and is lost from the land surface hydrologic cycle. The remaining 30% is used for industrial and domestic purposes; this water may return to the natural terrestrial hydrologic cycle after "consumption" of the water's low temperature (e.g., for power plant cooling) or gravity (e.g., for hydropower applications), or after the water quality has been improved (e.g., at sewage treatment plants). As a result, in terms of total global consumption, the estimated consumption of water—not the withdrawal of water—by agriculture is approximately 90% (Shiklomanov 2000). Thus, agriculture is the heaviest user of water, although the exact percentage of agricultural consumption is uncertain and requires further examination.

In which geographical environment do people use the most water for agriculture? Here, the key factors are abundant sunshine and high temperatures. In a warm, sunny environment, more water generally results in more agricultural crops. An environment with abundant sunshine and high temperature is also preferable for domestic and industrial activities. Thus, water in such an environment is often exploited to the maximum extent. Modern technology allows for rapid exploitation of water; people use more water than is renewable to get the maximum yield from the land.

Figure 16.2 shows high water stress under the above conditions. More specifically, each high water stress region is considered to suffer from one of the following types of water crisis:

1. Available water is physically scarce, such that people in the region require more water for their survival and development, and to improve their quality of life.

2. The aquatic ecology in the hydrosphere is significantly damaged because considerable amounts of water are regulated and/or diverted from rivers, ponds, and lakes.

3. Human society and natural ecology are apparently sustained, but sig-nifi cant amounts of water are withdrawn from groundwater aquifers, particularly from fossil groundwater aquifers.

Typically, the first crisis type occurs in economically and technologically poor regions, where the focus is on the development and stability of human society. The second type occurs in regions where the infrastructure is sufficient. The ecology is literally threatened; then again, the ecology has generally been a secondary priority throughout human history. The third type occurs in regions where sufficient technologies (e.g., high-performance pumping and power generation) are available. The society and environment may be sustained at the moment, but only superficially. The decline of the groundwater table, particularly the exhaustion of fossil water in the region, is the critical issue for sustain-ability. Fossil water is defined as groundwater in aquifers having slow recharge rates and mean water residence times at geologic timescales. The critical issue for the third crisis type is presumably similar to the critical issues of other resources, such as minerals, where the stock is the vital target of management for sustainability.

Sometimes, actual water crises result from a combination of three types. The famous disappearance of the Aral Sea is an example of a combination between the first and second types. Another well-known incident, the drying-up in the lower reaches of the Yellow River at the end of the twentieth century, was probably a combination of all three crises, along with climate change (Tang et al. 2008). Meanwhile, for the high water stress of the Colorado River, the highly developed economy and technology of North America means that the second type, ecology, must be dominant. A typical example of the third crisis type is the decline of the water table in the High Plains Aquifer of the Midwestern United States.

Thus far, description has been limited to so-called "blue water" issues, where blue water is defined as the water in rivers, ponds, lakes, and ground aquifers that can be withdrawn with relative ease for human activities. Conventionally, blue water tends to be the only target of water resources management by water engineers and planners. Two new concepts for water types, "green" and "virtual," have recently emerged in water resources management. There are several variations in the definition of green water; generally, it refers to either evaporation (evapotranspiration) or soil moisture from or in the land surface. Specifically, green water is soil water found mainly in the unsaturated zone; it originates from precipitation before it finally evaporates into the atmosphere. (Note, however, that soil water that originates from irrigation water is categorized as blue water.) People cannot directly manipulate green water. Green water is attached to the soil by its nature and is essential for nonirrigated farming, which occupies 80% of all farming areas. Roughly 30% of all the terrestrial evaporation (evapotranspiration) is estimated to be already utilized for human activities (Oki and Kanae 2006). Therefore, applying the green water concept, land and water management are closely related. It should be noted that blue and green water cannot easily be added. These two water types are not independent terms in the water budget equation. Rather, they represent two different viewpoints of the water cycle.

Virtual water (Allan 1998) is a newly developed concept defined as the volume of water virtually needed to produce commodities that are imported into a country. A similar, recent concept is the "water footprint" (Hoekstra and Chapagain 2008). Nowadays it is general practice for a country to import various products and foods produced in foreign countries; this is equivalent to a virtual importation of foreign water resources. The water demand for food and industrial production in a water-scarce region can be offset by importing food or industrial goods into the region because the weight of food and goods is much lighter than the weight of the water required for producing those goods. Thus, trade in "virtual water" is much easier and more effective than trade in real water. The assessment of sustainability becomes more complicated when virtual water is considered because all related international trade should be taken into account, even for the sustainability assessment of a specific region. The current total international virtual water trade is estimated to be about 1000 km-3 yr-1 (Oki and Kanae 2004), although only a part of it is used to compensate for water shortages.

Water footprint is similar to virtual water. A water footprint is the volume of water consumption required to produce commodities, products, and foods in an exporting country. The quantitative difference between virtual water and water footprint provides us with the amount of water virtually saved through import and export. However, as just stated, a part of it is used to compensate for water shortages, and this difference is relatively minor. In addition, water can be virtually imported and/or exported, but a water crisis is attached to each site. Usually, social or ecological problems appear at each site. Because we are able to view many crises worldwide, the term "global water crisis" is sometimes used.

Measuring Water Stress

Water Availability

Assessing water stress on a global level (Figure 16.2), as accurately as possible, is the f rst step in measuring and monitoring the global sustainability of water resources. Initially, the estimation of water availability requires accurate measurement of all fluxes and storages in the global terrestrial hydro-logic cycle (e.g., river discharge, precipitation, evaporation, soil moisture). Even in the current era of satellites and high-performance computers, however, there is nonnegligible quantitative uncertainty in the global hydrologic cycle. Hanasaki et al. (2008a) noted significant differences among, and uncertainty in, various major estimates of global river discharge. River discharge generally corresponds to water availability and is one of the most fundamental pieces of information for water stress assessment. The scatter of discharge among several estimates on global or continental scales is approximately 10%. This value is probably the result of calibration against a certain observation-based data set. Postel (1996), for example, reported scatter of approximately 40%. The two latest major data sets of global terrestrial hydrologic fluxes and storages (Oki and Kanae 2006; Trenberth et al. 2008) differ in their estimates of precipitation, evaporation, soil moisture, and other quantities. The precipitation amount is the most basic variable of the terrestrial water cycle. However, even the total precipitation amount in Japan, which probably has the densest network of rain gauges and radar and satellite information in the world, has at least a 10% uncertainty (Utsumi et al. 2008). Global uncertainty appears much greater than this, even in the latest estimates (Hirabayashi et al. 2008b; 2008c) due partly to the wind-induced undercatch. Thus, the estimates of the mean fluxes and storages of the hydrologic cycle averaged over several decades have significant uncertainty. It is even more difficult to obtain accurate information on temporal variability over decadal, interannual, and shorter timescales. Obtaining information on the extremes of hydrologic fluxes and storages is also extremely challenging.

Future water availability is likely to be affected by anthropogenically induced climate change, or "global warming." The latest summary of climate change assessments, the Fourth Assessment Report (AR4) of the IPCC (2007d), adopted several standard scenarios of future greenhouse gas emission in the Special Report on Emissions Scenarios (SRES). The scatter of projected changes in the hydrologic cycle among these scenarios is considerably large (e.g., Arnell 2004) because the differences in projected surface temperature change are large (ranging on the order of+1.5 to +4.5 K). Therefore, the projected change in populations under high water stress depends on the adopted future scenario (Arnell 2004; Oki and Kanae 2006) (Figure 16.3), although it is partly due to the fact that water withdrawal change rather than water availability change is the major controlling factor (discussed further below). In addition, even if the adopted future scenario was the same, there are considerable uncertainties in the future hydrologic cycles projected by climate models (Milly et al. 2005; Waliser et al. 2007).

Hydrologic extremes that have low occurrence probabilities (e.g., floods and severe droughts) must be considered in the assessment of future sustain-ability. However, only a few studies have begun to consider future global/continental projections (e.g., Milly et al. 2002; Lehner et al. 2006; Hirabayashi et al. 2008a). How these extremes should be incorporated into a water stress (and more generally, sustainability) assessment remains a topic for future research. In addition, the validation of simulated hydrologic extremes against observations

(a) W/Q > 0.4 (b) Q/capita < 1000 m3/capita/yr

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